Heretofore, interconnecting wires for semiconductor devices have been comprised of aluminum alloys, e.g., Al-1% alloy. These interconnecting wires are bonded by wedge bonding or ball-bonding. According to the wedge-bonding method, the wire is placed on the part to be connected and then pressed by a wedge with heating or ultrasonic excitation. According to the ball-bonding method, a small ball is formed at the end of a wire by means of an oxygen-hydrogen torch or arc discharge, and the thus-formed ball is pressed against the part to be connected. The ball-bonding method is widely used because of its high efficiency. The interconnecting wires used for ball-bonding is usually a gold wire which is superior in ball-forming performance.
On the other hand, the electrode of an element, e.g., an IC chip, is covered with of aluminum or an aluminum alloy because of its conductivity and ability to bond to the semiconductor. Thus, a connection is established between different metals, namely, aluminum and gold. The aluminum-gold connection tends to become brittle because an intermetallic compound forms during heating in the subsequent process. Therefore, the aluminum-gold connection is less reliable than an aluminum-aluminum connection. In addition, the former is not desirable because of the high price of gold. Under these circumstances, there has long been a strong demand for an interconnecting wire comprising an aluminum alloy which can be used for ball-bonding. The conventional interconnecting wires comprising an aluminum alloy cannot be used for consistent ball-bonding these wires have poor ball-forming performance.
Interconnecting wires used for semiconductor devices are required to have another function. For example, interconnecting wires used for power transistors are required to function as fuses when an excessive current is applied by accident. (Such an excessive current might damage not only the power transistor but also the entire electronic apparatus containing other semiconductor devices.) Unfortunately, conventional interconnecting wires made of aluminum are not readily fused by an excessive current, because their melting point is 660.degree. C. even if they are made of high-purity aluminum. On the other hand, a low-melting metal such as solder is not suitable for use as the interconnecting wire of power transistors because of its low conductivity, poor workability, and insufficient strength. For these reasons, circuits for semiconductors are usually provided with a separate unit that functions as s fuse.
Interconnecting wires for semiconductors may be given the function of a fuse by (1) reducing the wire diameter, (2) increasing the electric resistance, or (3) lowering the melting point. The first method has the disadvantage that the same bonding machine cannot be used for both a wire of reduced diameter and a wire which need not function as a fuse. The second method has the disadvantage that the wire has to have a large diameter to attain the desired conductivity. This poses the same problem as in the first method. As for the third method, no aluminum alloy having a sufficiently low melting point, e.g., about 300.degree. C. has been obtained yet as far as is known although the coexistence curve between solid and liquid of the equilibrium diagram of aluminum is shifted by the adition of various elements as described in, e.g., Hansen; "Metallography Structure and Phase Diagram." Metal Handbook, vol. 8.